November 7, 2011

Organic Light-Emitting Diodes

by editor

Research highlights energy efficient solutions for display and lighting applications

The search for innovative, cost-effective and energy-efficient solutions for display and lighting applications are the focus of ongoing intense worldwide photonics research and development activities. Organic light-emitting diodes (OLEDs)–lightweight, potentially flexible, cheap-to-fabricate and highly energy-efficient lighting and display devices–offer one solution in alternative energy lighting and display applications. To highlight breakthroughs in this area, the Optical Society (OSA (http://www.osa.org)) today published a special Focus Issue on OLEDs (http://www.opticsinfobase.org/oe/issue.cfm?volume=19&issue=106) in Energy Express (http://www.opticsinfobase.org/ee), a bi-monthly supplement to its open access journal Optics Express (http://www.opticsinfobase.org/oe/home.cfm). The issue is organized and edited by Guest Editors Emil J.W. List of the NanoTecCenter Weiz GmbH and Graz University of Technology in Austria, and Norbert Koch of the Institute of Physics, Humbolt University in Germany.

"OLEDs are amongst the most promising candidates for alternative display and lighting solutions," said List. "The exciting findings presented in this focus issue will ultimately translate into real-world applications, providing consumers with cost-effective technology while reducing electricity consumption."

More than 20 percent of the world's total electricity consumption is used for lighting applications. That number increases to an estimated 25 percent when including display and TV applications. OLEDs, along with inorganic solid-state lighting technologies, are considered to be on the forefront of 21st century display and lighting technologies. The widespread use of this technology could save hundreds of gigawatt hours (GWh) of power or millions of tons of coal per year.

"OLEDs can be found in a variety of everyday products such as television screens, computer monitors and smartphones," said Koch. "As use of these products becomes more widespread, the need for research and development also grows. The latest advances reflected in this focus issue are truly exceptional and will prove to be invaluable to advancements in lighting and display technology."

Key Findings and Select Papers

The following papers are some of the highlights of the Energy Express Focus Issue on OLEDs. All are included in Volume 19, issue S6 and can be accessed online at http://www.opticsinfobase.org/ee.

The outcoupling of light from an OLED may be tackled by different means, including optical feedback structures in the active layer, by high index-media in top-emitting OLED, by lens or microlens-like arrays, or by using microcavity effects in the active device. As reviewed and discussed by Simone Hofmann, Karl Leo and their colleagues from the Institute for Applied for Photophysics, TU Dresden in Germany, in particular top-emitting OLEDs seem to be beneficial for lighting and display applications. Here, non-transparent substrates are used. The authors review and discuss different optical effects of the microcavity structure and identify important loss mechanisms due to waveguiding and surface plasmons, and show that further improvement in light extraction is required to reach the targeted high outcoupling efficiencies.

A very practical approach to improve the outcoupling efficiency in OLEDs up to 60 percent is demonstrated by Ruth Shinar and Joseph Shinar from the U.S. Department of Energy's Ames Laboratory and Iowa State University, and coworkers, using index-matching microporous phase-separated films of polymer blends acting as random microlens-like arrays. They demonstrate that the use of such blended thin films provides an economical method independent of the OLED fabrication technique, for improving outcoupling.

To overcome the losses at the organic layer/cathode interface and to optimize the optical path in the devices, Lian Duan, Yong Qiu and their colleagues from the Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, and the R&D Center, Visionox Tech. Ltd, Beijing, introduce an approach by using a novel, rather thick, n-doped layer. Using a combination of a low-temperature-evaporable n-dopant KBH4 and a high charge carrier mobility electron transport material they show excellent performance of their devices due to reduced losses at the organic layer/cathode interface.